A. Field of the Invention
This invention lies in the field of improving the damping action for loudspeakers and acoustic membranes which respond to electrically actuated voice coils movable to reproduce the entire frequency range and the desired intensity of acoustic vibrations and in particular relates to improvements in both the response of the diaphragm to the voice coil throughout the entire frequency range and to the heat dissipation from the voice coil.
In connection with this improvement in acoustic response and in heat dissipation, the invention also relates to novel means which prevent dust contamination effecting the voice coil and thereby impairing the operation of the voice coil when damping means in the form of a ferromagnetic fluid is applied to the voice coil.
B. Description of the Prior Art
1. The Use of Liquids in the Transducer PA0 2. Use of Liquid Suspensions Containing Finely Divided Solids PA0 3. The Commercial Development and Use of Magnetic Fluids
Fluids have been used heretofore to exhibit transducing action at a meniscus in response to changes in an electrical field and an example of such fluid is mercury used to convert electrical potential directly into liquid pressure as shown in Burgess, U.S. Pat. No. 2,416,978 of Mar. 4, 1947. However, this Burgess device does not provide a damping function nor is there a heat sink.
Haines et al, U.S. Pat. No. 3,304,378, shows an electrical field effect transducer in which a suspension of silica in oil is used as a shear-responsive fluid confined between opposed conducting surfaces and which serves as a frequency doubling sonic transducer.
Klass, U.S. Pat. No. 3,385,793, discloses electroviscous fluids which respond to the influence of electric potential by evidencing an apparent pronounced increase in bulk viscosity. A number of such fluids are also described in U.S. Pat. Nos. to Willis M. Winslow, 2,661,596; 2,661,825 and 3,047,507 and are useful in clutches, wherein the fluids are disposed between the surfaces of two electrically conductive members, and an electric potential is imposed across the two members. The fluid responds to the application of electric potential by instantaneously but reversibly changing in bulk viscosity.
Flame-resistant hydraulic fluids have been disclosed in U.S. Pat. No. 3,567,644 to Hotten and these fluids are used with thickeners or suspending agents to be operable for power transmission in a wide range of temperatures. These fluids are compared with phosphate materials which have low viscosity indices to require substantial amount of viscosity index improvers and have also been compared with chlorinated waxes which require chlorination of at least 40.degree. to achieve flame-resistance. However, these chlorine containing materials are dangerous because they give a lot of fumes.
Special phosphates have been described in Attwood, 3,074,889 which are modified with flame-proofing agents.
Yamamuro et al, U.S. Pat. No. 3,828,144, shows a vibration absorbing support for loudspeaker voice coil bobbin in which the vibration absorbing members are formed of silicone rubber. Williams et al, U.S. Pat. No. 2,444,620 and Barker, U.S. Pat. No. 2,164,374 each teach elastic resilient suspensions for the diaphram.
In Machine Design, June 1, 1972, there appears an article entitled "Magnetic Fluids" by Francis J. Lavore, which describes a colloidal suspension of ferrite particles about 100 Angstroms in size suspended in a non-volatile liquid such as a hydrocarbon, a silicone or a fluorocarbon which is used as a seal. The material for this seal is identified under the Trademark "FERROMETIC", manufactured by the Ferrofluidics Corporation, 144 Middlesex Turnpike, Burlington, Massachusetts 01803 and the "Newsreports" section in Electrimechanical Design, October 1973 describes the use of Ferrofluids to seal the space between the poles of a loudspeaker.
The magnetic fluid seals which are described in U.S. patents by the Ferrofluidics Corporation are the following:
______________________________________ Name Pat. No. Title ______________________________________ Rosensweig 3,612,630 Bearing Arrangement With Magnetic Fluid Defining Bearing Pads Rosensweig 3,620,584 Magnetic Fluid Seals Rosensweig et al 3,648,269 Magnetic Fluid Display Device Rosensweig 3,734,578 Magnetic Fluid Pneumatic Bearings ______________________________________